This invention relates in general to electrically controlled hydraulic control devices for a lifting unit of a mobile agricultural machine, particularly a farm tractor having a frame and a control rod pivotally secured to the frame by a pin, the rod being coupled to the lifting unit to move between an elevated position and a lowered position. Electrohydraulic control devices of this type are known, for example from U.S. Pat. No. 3,814,188, wherein each lower control rod of an electrohydraulically controlled three-point linkage is pivotally secured to the body of the tractor by means of a bending rod. Each bending rod has a cylindrical section between its rigidly clamped end and its free end supporting the lower control rod, whereby strain measuring straps are arranged on the upper surface of the cylindrical section to apply electrical signals proportional to forces transmitted by the lower control rods to an amplifier. Both bending rods together with their parts for supporting the strain measuring straps are arranged in the interior of a protective tube rigidly secured to the body of the tractor. The disadvantage of this prior-art control device is the fact that the construction and arrangement of the load-sensing device is very costly. In particular, there are necessary expensive bending rods made of a high quality material, and in addition the rods require a considerable installation space. Furthermore, the assembly of the load-sensitive gauge is difficult. In the case of a defect on the load-sensing gauge, the lower control bars as well as the protective tube together with the bending rods must be dismanteled. Accordingly, the installation of replacement parts is expensive as regards both installation time and material expenses. Also the leading out of electrical connection cables from the load-sensing straps to the electrical amplification circuits via bores in the protective pipe is relatively prone to interferences and cumbersome.
From the British Pat. No. 1,068,336 an electrohydraulic control device for operating a three-point linkage in a tractor is known, in which the load in the lower control rod is detected by means of strain-measuring straps. For this purpose each lower control rod is formed with an annular groove, the wall of which supports four circular strain-measuring straps connected in a bridge. The disadvantage of this solution is again the application of strain-sensing straps which are relatively sensitive to interferences. The mounting, interconnection and protection of the strain-sensing straps is time-consuming, predominantly manual operation. In addition, the straps are arranged in the movable parts of the tractor. As a result, the measuring signals are difficult to detect and the lower control rods supporting the strain-measuring straps must have a special design to enable the support. In the case of a defective strain-measuring pick-up, it is frequently necessary to exchange the entire lower control rod. Furthermore, the construction according to this British patent is difficult to assemble and is poorly suited for rough operational conditions of agricultural vehicles.
Furthermore, in the U.S. Pat. No. 3,516,498, an electrohydraulic control device for lifting units of a tractor is described, which cooperates with the lower control rod of the tractor. In this design, the lower control rod is linked via a rocking lever to the body of the tractor and additionally is linked to an elbow lever linkage. In order to measure loads taking place in the lower control rod there is provided an elbow lever link supported by a spiral spring and cooperating with an inductive displacement pick-up. The disadvantage of this solution is, in the first place, the high cost for the construction for measuring the load of the lower control rod.
From the publication of C. H. Rohrbach, "Handbuch fur elektrisches Messen mechanischer Grossen," 1967, it is known how to employ magnet or elastic transformer pick-ups for measuring torques on a shaft. This transformer pick-up includes a stationary first magnetic core with primary coils and secondary coils, the core being applied to the shaft. Due to the tensile and compressive strains occuring in the shafts during the transmission of the torsional moment, the permeability of the shaft material alters and so alters the coupling between the coils of the pick-ups. From those coupling changes a measuring voltage can be derived that is proportional to the torque transmitted by the shaft. This known transformer pick-up, however, is limited to the measurement of torsional moments.